Ian,
I found that was a chronic problem in the exhaust manifold of my'36 Chevy pickup when I restored it in the early 1970s and others I spoke with were having the same issue. The hammering and penetrating oil did solve the problem temporarily but it kept happening again.
If you decide to split the manifolds as Matt suggests it's only a little more work to make a permanent fix. If you put stainless steel bushings in the manifold and make a heat riser shaft from stainless steel you won't ever have to deal with this again. I did that on my '36 PU about 35 years ago and the heat riser valve still functions perfectly without any "persuasion".
I don't know how cold it gets in Tennessee but on cold mornings a functional heat riser valve does help.
